This invention relates to a fired heater used in hydrocarbon processing, and in particular to a fired heater system.
There are many designs for fired wall heaters. Fired heaters provide a high heat flux to rapidly heat process streams to high temperatures. A common type of heat exchanger that is used in conjunction with the fired heater is a shell and tube exchanger, where heat is exchanged between a fluid flowing through the shell and a fluid flowing through the tubes. Two common designs include a hot combusted gas that is passed to a shell volume and heats up a process fluid flowing through tubes passing through the shell volume, and a process fluid flowing through a shell volume with the hot combusted gas flowing through tubes passing through the shell volume. For shell and tube heat exchangers, when the heating medium is hot gas, the heating medium is generally directed to the shell side of the exchanger, and when the heating medium is a hot liquid, the medium is generally directed to the tube side of the exchanger.
There are many designs for shell and tube heat exchangers, which include features for allowing multiple passes of a fluid in the tubes, supports for the tubes in the shell, and designs for easy maintenance of the exchanger. Many aspects of these designs are to accommodate the expansion and contraction of the materials in the exchanger during process cycles, while maintaining the exchanger integrity.
These heaters are used for a variety of processes from vaporization of high boiling point liquids to thermal cracking of hydrocarbons to thermal reforming processes to pyrolysis of hydrocarbon materials. The heaters often have a pre-heat section, followed by a radiant heating section where the temperatures rise to in excess of 500 C. The heat fluxes in the radiant heating section is subject to the heat shadows created by neighboring tubes, or the structure of the heat exchanger leading to differential heat fluxes.
The heat flux in these heat exchangers can be substantially non-uniform and can lead to hot spots on the exchanger. The hot spots can lead to coke production in hydrocarbon rich streams. The coking leads to further hot spots and can lead to a shortened on-stream time for the heat exchanger and increase maintenance costs.